Semiconductor laser arrays are monolithic laser devices having a plurality of spatially displaced emitters in an active layer of a semiconductor body acting as a waveguide for light wave propagation under lasing conditions. In such laser arrays it is desired to mutually couple the waveguides together so that light waves in each waveguide are at the same wavelength and radiate from a front facet of the laser array device in-phase with each other to thereby reduce beam divergence. One well known means for coupling is provided by reducing the separation between adjacent waveguides sufficiently to achieve optical wave overlapping or "evanescent coupling" of light waves propagating in adjacent waveguides.
A second manner of coupling, called "direct coupling", is described by Scifres et al. in U.S. Pat. No. 4,255,717 and reissue patent No. 31,806. In this laser device 10, shown in FIG. 7 herewith, layers 14, 16, 18 and 20 of GaAlAs or other semiconductor materials are deposited on a substrate 12. Layer 16 with the highest index of refraction and lowest bandgap is the active layer, providing a waveguide for light wave propagation under lasing conditions along the plane of the p-n junction 22. A plurality of waveguides 33 is defined by a current confining channel geometry comprising zinc diffusion 28 through a blocking layer 20 and parallel contact stripes 32, for example. Other techniques for defining the current paths and thereby the waveguides such as proton implantation are also described. The waveguides are directly coupled by interconnecting stripes 38. Other light coupling structures are also described. Light wave portions propagating in one waveguide will split off and be deflected into an adjacent waveguide where they will combine with light wave portions propagating in that adjacent waveguide. Out-of-phase modes of the light waves will radiate out of the waveguide junction and be lost when the waveguides are recombined. Only the mode in which all emitters are in-phase can oscillate, and the resulting emitted light 40 will be coherent and have a smaller beam divergence.
A problem with semiconductor laser arrays which employ mutually coupled waveguides is that if one or more waveguides cease to lase, the entire device may no longer operate in-phase. For example, in the directly coupled laser array seen in FIG. 7, if any single branch of an emitting cavity, such as the one labeled A, no longer lases, the coupling between the two parts of the array will be broken. Light wave portions propagating in waveguides on one side of the breach will be in-phase, as will light wave portions propagating in waveguides on the other side of the breach, but the light wave portions on each side may not be in-phase with each other.
Accordingly, it is an object of the present invention to provide a semiconductor laser array which remains coupled in-phase when one or more lasing branches fail to operate.